Conventional fluorine-based greases have been composed by adding small amounts of various additives, such as rust preventing agents, to thickening agents, such as a homopolymer (PTFE) of tetrafluoroethylene (TFE), a copolymer (FEP) of TFE and hexafluoropropylene, a copolymer (PFA) of TFE and perfluoroalkyl vinyl ether, and a copolymer (ETFE) of TFE and ethylene, with perfluoropolyether as a base oil, and have been used under severe conditions that require low-temperature properties, high-temperature durability, oxidation stability, chemical resistance, and the like.
However, since both base oils and thickening agents are fluorine-containing polymers, the conventional fluorine-based greases are expensive and problems such as poor fit with resins, metals, rubbers and the like, which are the materials to be lubricated, the fact that the oil film necessary for lubrication cannot be formed under conditions such as high load, causing abrasion and poor torque transmission efficiency due to a high friction coefficient, as well as poor rust prevention and corrosion resistance arise.
In order to solve such problems, it has been proposed to use a mixture obtained by mixing non-fluorine-based grease with fluorine-based grease. For example, in Japanese Patent Application Disclosure No. 7-268370, there is disclosed a grease containing hydrogenated mineral oil and/or synthetic lubricating oil, a fluoropolyether oil and an organic or inorganic thickening agent, wherein the weight ratio of lubricating oil+fluoropolyether oil:thickening agent is 97:3 to 80:20 and the weight ratio of lubricating oil:fluoropolyether oil is 95:5 to 60:40.
It has been considered that a homogenizer such as a Manto Galvin type homogenizer or a three-cylinder homogenizer (the three-cylinder homogenizer can be thought of as being a three-division cylinder block type homogenizer) is used for the blending for preparing greases using such base oil mixtures and the number of homogenizer treatments is preferably two to three times the number of treatments performed for an ordinary non-fluorine-based grease, in order to obtain superior homogeneity. However, it is difficult to obtain a homogeneous grease mixture by the blending treatment with a homogenizer even if the number of treatments is increased.
Although in Japanese Patent Application Disclosure No. 2003-96480 and 2006-182923 which concern applications by the applicant, there are disclosed a method of manufacturing a lubricating grease composition comprising a non-fluorine-based grease and a fluorine-based grease by thoroughly kneading them with a three-roll or high-pressure homogenizer, no mention is made of obtaining a homogeneous grease mixture.
Although a grease mixture is not only less expensive than a single fluorine-based grease, but also superior in abrasion resistance to an opposite material, there is no mention of a homogeneous dispersion of not mutually compatible base oils, and the base oils of a grease may be separated depending on the dispersion degree.